南中國海東北部過去三萬八千年以來的古海洋變化;Paleoceanographic change of the northeastern South China Sea during the last 38,000 years

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Title:	南中國海東北部過去三萬八千年以來的古海洋變化;Paleoceanographic change of the northeastern South China Sea during the last 38,000 years
Authors:	王子軍;Wang, Tzu-Chun
Contributors:	地球科學學系
Keywords:	古海洋;氧同位素;有孔蟲;南中國海
Date:	2020-08-13
Issue Date:	2020-09-02 15:39:28 (UTC+8)
Publisher:	國立中央大學
Abstract:	南中國海東北部有著快速的沉積速率，為古海洋研究提供了高年代解析度的沉積物紀錄。本研究藉由採集於南中國海東北部的海洋岩心MD18-3568進行古海洋研究，由有孔蟲殼體的碳十四定年建立岩心年代模式，使用棲息於不同水深的有孔蟲(Globigerinoides ruber, Neogloboquadrina dutertrei, Uvigerina spp.與Bulimina aculeata)殼體氧同位素來重建古海洋變化。海洋岩心MD18-3568位於台灣西南外海的南永安西海脊之東翼，水深為1,315公尺，岩心總長20.7公尺。根據通過岩心站位的高解析海床底質剖面，顯示此處沉積物為水平堆積，且岩心由富含生物殼體的泥質沉積物組成，因此判斷此岩心為懸浮堆積的半遠洋沉積物。碳十四定年結果顯示，岩心底部的年代為距今約37,000年前，並且年代往上依序變年輕、沒有年代倒轉現象，代表此岩心為古海洋研究的良好材料。 G. ruber(棲息於表層水的種屬)與N. dutertrei(棲息於次表層水的種屬)的氧同位素差值反映表層水的溫度與鹽度變化，而表層水的溫度與鹽度在此區域主要受到夏季季風控制，故本研究以此差值(δ18Odut-δ18Orub)當作東亞夏季季風強度的指標。結果顯示，在海洋同位素階段3(Marine Isotope Stage 3, MIS 3)與全新世有較大的差值，代表表層水呈現高溫低鹽的環境，使表層水與次表層水之間分層較明顯，可以對應到較強的夏季季風；而在末次最大冰期(Last Glacial Maximum, LGM)有較小的差值，代表南海東北部的表層水與次表層水有較好的混和，可以對應到較弱的夏季季風。N. dutertrei(浮游性有孔蟲)與底棲性有孔蟲的氧同位素差值反映著次表層水的溫度與鹽度變化，而黑潮水的侵入會使南中國海東北部的表層水與次表層水鹽度增加，故本研究以此差值(δ18OUvi-δ18Odut)當作黑潮水侵入的指標。結果顯示，在過去37,000年差值並沒有太明顯的變化，代表黑潮水的入侵在這段時間內並沒有太大的改變。 ;The northeastern South China Sea (NE SCS) shows high sedimentation rate, and thereby it offers scope for high-resolution paleoceanography study. This study is based on AMS 14C dating on planktonic forams and oxygen isotope data of different foraminiferal species (Globigerinoides ruber, Neogloboquadrina dutertrei, Uvigerina spp. and Bulimina aculeata) from the sediment core MD18-3568, to reconstruct the paleoceanographic change of the NE SCS. The marine sediment core MD18-3568 is located in the accretionary wedge offshore southwest Taiwan and it situates at the South Yungan West Ridge at a water depth of 1,315 m. The whole core is dominated by hemipelagic sediments and is 20.7 m in length. AMS 14C ages show that the bottom of this core is around 37,000 years BP, and there is no age reversal for 16 dated horizons, indicating that the core is ideal for high-resolution paleoceanographic study. The δ18O offset between G. ruber (surface water dwelling species) and N. dutertrei (subsurface water dwelling species) is influenced by the freshwater input and the temperature different between surface and subsurface water. We infer that it can be caused by the summer monsoon in NE SCS. During the Last Glacial Maximum (LGM), the δ18O offset between G. ruber and N. dutertrei was smaller, indicates weaker summer monsoon; the offset became larger during Holocene, indicates stronger summer monsoon. The δ18O offset between N. dutertrei (planktonic species) and benthic foraminiferal is influenced by salinity and temperature different between subsurface and bottom water. In our study area, the Kuroshio intrusion will lead to higher salinity for the surface and subsurface water, so we infer that (δ18OUvi-δ18Odut) value may be used as an indicator for the strength of the Kuroshio intrusion into the NE SCS. The δ18O offset between N. dutertrei and benthic foraminiferal didn’t show significant variation, this indicates that the strength of the Kuroshio intrusion didn’t change a lot during the last 37,000 years.
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